Effect of low-pressure carburizing temperature on the fatigue properties of 17Cr2Ni2MoVNb and 20Cr2Ni4A gear steels

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In this work, the fatigue properties of 17Cr2Ni2MoVNb and 20Cr2Ni4A gear steels after low-pressure carburized at 930 °C and 980 °C were investigated by means of rotating bending fatigue tests. The results showed that at the carburizing temperatures of 930 °C and 980 °C, the fatigue limits of 17Cr2Ni2MoVNb steel were 977 MPa and 967 MPa, while those of 20Cr2Ni4A steel were 967 MPa and 865 MPa, respectively. The fatigue limit of 20Cr2Ni4A steel evidently decreased at high temperature carburizing, and the fatigue crack initiation site was also shifted from non-metallic inclusions to the matrix. It could be attributed to the abnormal growth of grains in the carburized layer, which might weaken the grain boundaries and lead to a change in the cracking mode. On the other hand, the grain sizes of 17Cr2Ni2MoVNb steel after low-pressure carburizing at 930 °C and 980 °C were both fine, thus resulting in similar fatigue limits. Under high stress amplitude, the fatigue strength of 17Cr2Ni2MoVNb steel was higher than that of 20Cr2Ni4A steel for the same number of cycles, which is related to the amount of retained austenite transforming into martensite during fatigue tests.

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